The invention relates to a method for the power-shiftable changeover of a hydrostatic vehicle drive of a mobile construction machine, having at least one hydrostatic capacity pump, a preferably controlled hydrostatic drive motor and an at least two-stage power-shift transmission, which drives the axle(s) of the construction machine via its drive shaft, wherein the power shift transmission is shifted via at least one change-over valve, by means of pneumatic pressure, hydraulic pressure or electrical voltage.
The invention furthermore relates to a device for the power-shiftable change-over of a hydrostatic vehicle drive of a mobile construction machine, having at least one hydrostatic capacity pump, a preferably controlled hydrostatic drive motor and an at least two-stage power shift transmission, which drives the axle(s) of the construction machine via its drive shaft, wherein the power-shift transmission can be shifted via at least one change-over valve, by means of pneumatic pressure, hydraulic pressure or electrical voltage.
Mobile excavators or comparable construction machines, which have manually shiftable, two-stage spur-wheel drives in connection with a hydrostatic vehicle drive or which have only a hydrostatic vehicle drive (hydrostatic transmission) are known. The shifting occurs either through a manual shifting of the transmission by the driver or through automatic or manual shifting operations in the hydrostatic vehicle drive.
During the manual shifting of the transmission, it cannot be ruled out that the vehicle drive is not operated at the optimum shifting stage. In the most unfavorable case, this can even endanger the driving safety as well as the components of the construction machine, e.g. if a shifting to a low gear occurs at excessively high driving speeds.
German Patent 195 20 454 C1 from the assignee of the present invention discloses a method and a device for the load-dependent, automatic change-over of at least one two-stage hydraulic drive motor of a mobile construction machine. With this method and following the activation of a switch, at least two pressure switches that are operatively connected to a switching element are activated one after another with different hydraulic contact pressures. In the process, a valve that causes the drive motor to change over from its highest intake volume to its lowest intake volume and vice versa is electrically actuated by the change-over valve, which in turn is actuated by one or the other pressure switch in dependence on the preset load conditions in the hydraulic drive circuit. The known method therefore concerns an automatic change-over of a two-stage hydraulic drive motor, but does not concern a generic-type method of shifting a power-shift transmission.
It is the object of the invention to modify a method and a device of the generic type in such a way that an automatic and operationally safe change-over between stages of the power shift transmission in dependence on the load is possible, so that the total driving range from minimum to maximum speed can be traversed at different driving resistances without requiring a manual shifting. In addition, the driving safety is to be
This object is solved according to the invention with a method of the aforementioned type, in that by actuating a switch, at least two pressure switches that are operatively connected to at least one switching element are actuated sequentially with different hydraulic contact pressures for a load-dependent automatic shifting of the power-shift transmission. In the process, the switching element is activated by one or the other pressure switch, in dependence on preset load conditions in the hydraulic driving circuit, and this switching element in turn electrically actuates the change-over valve to effect the shifting of the power transmission.
According to the invention, the driver thus can effect an automatic shifting by actuating a switch. In the process, a switching window is generated with the aid of two pressure switches and one relay (switching element) or similarly operating devices, which switching window permits on the one hand the automatic shifting in both directions (stage 1 following stage 2 and stage 2 following stage 1) and, on the other hand, prevents unnecessarily frequent shifting. The shifting occurs in dependence on the hydrostatic pressure in the driving circuit (maximum or minimum value).
In order to increase the driving safety and the safety of the machine components during the shifting, a particularly advantageous embodiment provides that the driving speed of the construction machine is monitored and a shifting back of the power-shift transmission is prevented if a preset speed value is exceeded. This can be achieved particularly advantageously in that a frequency signal that is proportional to the driving speed is evaluated by an evaluation unit in order to monitor the driving speed, and that a frequency-dependent switch is used to make possible or prevent the shifting.
An unproblematic shifting to a low shifting stage is therefore possible only in cases where the driving speed of the construction machine permits this without a problem. For this, a corresponding unit evaluates a driving-speed proportional frequency signal (tachometer signal). A shifting can occur only if the driving speed (frequency) is within a predetermined shifting window.
According to this method, it is furthermore preferably provided that the frequency-dependent switch acts upon the change-over valve. Alternatively, it can also be provided that the frequency-dependent switch acts upon an additional valve, which is arranged parallel to the change-over valve and is hydraulically connected to this valve.
In addition, it is advantageously provided that the shifting up of the power-shift transmission occurs exclusively by means of the pressure switches and depending on the load. For this, the frequency-dependent switch must be connected via suitable elements, e.g. relays, to the change-over valve, so as to program the frequency-dependent circuit in such a way that it is only effective for the shifting down.
It is furthermore advantageously provided that the driving speed of the construction machine is monitored and an automatic shifting back to the lowest gear of the power-shift transmission occurs if the construction machine is stopped. It makes sense to do this via the frequency-dependent switch, which makes it possible to determine whether the vehicle is stopped or not. An additional frequency-dependent switch can also be provided for this.
According to yet another advantageous embodiment, the lower contact pressure for the pressure switches is adjusted such that it corresponds to a pressure of the capacity pump in the range of its maximum pumping capacity.
The method furthermore provides that the upper contact pressure for the pressure switches is adjusted so as to be in the range of the maximum system pressure.
In order to solve the initially defined problem, the generic device according to the invention is distinguished by having a switch and at least two pressure switches with different hydraulic contact pressures for the load-dependent automatic shifting of a power-shift transmission, wherein the pressure switches are operatively connected to a switching element that is connected to the change-over valve and electrically actuates this change-over valve.
The switch is normally located in the driver cab of the construction machine and effects the selection between manual and automatic shifting. The switching element that is connected to the pressure switches can preferably be designed as a relay.
A particularly advantageous embodiment of the invention provides for an evaluation unit for monitoring the driving speed and a therewith connected, frequency-dependent switch, which permits or prevents the shifting of the power-shift transmission.
It is advantageous in this case if the frequency-dependent switch is operatively connected to the change-over valve. As an alternative, it can also be provided that the frequency-dependent switch is operatively connected to an additional valve, which is arranged parallel to the change-over valve and is hydraulically connected to it.
It is particularly advantageous if the frequency-dependent switch has a first output, which is operatively connected to the change-over valve by way of a relay. With the aid of this relay, it is possible for the frequency-dependent and thus also speed-dependent shifting to become effective only if the speed is reduced, meaning during the down-shifting of the power-shift transmission. This is designed to avoid a shifting up, which does not depend on the load. As a result, it is ensured that the shifting up occurs only in dependence on the load.
It is furthermore advantageously provided that the frequency-dependent switch has a second output, which is operatively connected via an additional relay to the change-over valve, wherein the additional relay is arranged between the change-over element and the switch as well as the relay. By providing this additional relay, it is possible to effect an automatic shifting to the first gear if the vehicle is stopped.
The frequency-dependent switch can have two outputs, as described in the above. Alternatively, it is also possible to provide two separate switches with respectively one output inplace of the frequency-dependent switch.
In place of the previously described individual components, it can also be provided that the pressure switches, the switching element, the frequency-dependent switch and the relays form a joint electronic, hydraulic or pneumatic switching logic.